There are two distinct major fields of endeavor related to the utilization of chemicals effective to combat biological pests, whether plant or animal. The first is the discovery and development of chemical compounds and mixtures of such compounds which selectively adversely affect the pest without damaging the environment to be protected, particularly harvestable crops, livestock or ornamental plants. The other is the development of methods of applying appropriate dosages of such so-called "biologically active" compounds to the target host in an efficient and economical manner. Such methods include the use of formulations which include ingredients chemically inert to both this target host and the pest to be controlled as well as to the "active ingredient." These "inert" ingredients are effective in determining the physical characteristics of the total formulation.
A particularly useful formulation is one in which the "active ingredient" can be readily dispersed in water or a primarily aqueous medium. The end-user can then readily prepare suspensions and apply them to the target host in the appropriate dosage by controlling the amount of dilution with water. Ideally, the "active ingredient" should itself be readily soluble in water but unfortunately very few of the compounds of interest as pesticides are or can be altered to be sufficiently water soluble, at least not without adversely affecting their cost or biological efficiency. Therefore, the practice has developed of combining or formulating these compounds with "inert" ingredients to facilitate their dispersibility in water.
Such water dispersible formulations have generally been prepared by combining the "active ingredients," an emulsifier or wetting agent, a dispersant and an "inert" carrier or filler material. This last material has been found necessary in order to obtain a suitable admixture of the emulsifier, dispersant and active ingredients and to facilitate the size reduction of these ingredients to an appropriate particle size, e.g., in a hammer mill or ball mill. This carrier material may be either a solid or a liquid. The former type of formulations include wettable powders such as those discussed in U.S. Pat. No. 3,737,551, although other types of solid formulations such as the water dispersible granules of U.S. Pat. No. 3,920,442 have been proposed to overcome the disadvantages of handling fine powders. The latter type of formulations include flowables in which the major dispersing medium is water, such as those disclosed in U.S. Pat. No. 3,060,084. Typical of such products are SENCOR herbicide marketed by Mobay Chemical Corporation, SEVIN insecticide marketed by Union Carbide and FURADAN insecticide marketed by FMC.
Formulations are also available in which the "active ingredient" is dispersed in a petroelum distillation product. Normally, such formulations are not dilutable with water but find application in low or ultra-low volume applications. Typical of such products are SEVIN oil insecticide marketed by Union Carbide and DYLOX 1.5 oil insecticide marketed by Mobay Chemical Corporation. The latter product is interesting in that its active ingredient is also available in a water soluble powder formulation.
Not all pesticidally active compounds are amenable to formulation in a water-based flowable composition. In particular, the compound may not have long term stability in the presence of water but may hydrolyze or it may be difficult to prepare stable dispersions with suitable viscosities. In such cases, the compound may be dissolved in an organic solvent which also contains emulsifier but, in such cases, the stability of the solution becomes of concern. If the concentration of the solution is sufficiently high (and the economic incentive is to minimize the biologically ineffective solvent), crystallization may occur at normally encountered field temperatures and redissolution may not be readily achieved. Such crystallization may also occur when the formulation is combined with cold water resulting in plugged feed lines or spray nozzles in the application equipment. In addition, these solvent diluents may render the formulation incompatible with other pesticides with which it may be desirable to apply it. Finally, many of the solvents suitable for many active compounds of interest pose environmental problems.
As discussed hereinabove, formulations have been prepared in media in which the active compounds are almost totally insoluble (less than 100 ppm is usually recommended) but unfortunately for many such compounds, these media are not readily emulsifiable with water. An interesting exception is the formulation of Sevin insecticide and a dispersant in molasses which while being water miscible is believed to have no solubility for the active compound. On the other hand, many of the media which do display suitable compatibility with water and particularly those which are miscible with water also display sufficient solvation for active compounds of commercial interest to give rise to the problem of "Ostwald ripening." This phenomenon, beneficially utilized in the field of photographic emulsions, results when the smaller particles in a suspension dissolve in the suspending medium and then crystallize out on the larger particles, thus increasing the average particle size of the suspension until it ultimately becomes unstable and precipitates or separates from the medium. However, a formulation of what is believed to be metribuzin and lignin sulfonate dispersant in a 6:1 water/propylene glycol medium has been introduced ito the market and appears to form a stable dispersion in spite of metribuzin's solubility of 1200 ppm in water.
A technique has now been discovered to suppress this phenonmenon and prepare stable suspensions of active compounds in media in which they have significant solubility. Thus, these compounds can be suspended in media which are readily emulsifiable with water or even miscible with water.
The media utilized in the present invention have been used as carriers or diluents for biologically active ingredients. U.S. Pat. No. 4,000,258 discloses suspensions of an insecticidally active microorganism in various alcohols and, in some cases, these suspensions also include surfactants which aid water dispersibility or which reduce the suspension's tendency to settle. While the suspended ingredient is present in finely divided form, its solubility, if any, in the carrier is not reported. An article in Toxicology and Applied Pharmacology, Vol. 47, pages 451 to 460 (1979) reports the use of propylene glycol to "suspend" a variety of insecticides for a 24-hour exposure test. Some of the "suspended" insecticides are reported to be miscible with propylene glycol while others are not reported to be soluble in this solvent. There does not, however, appear to be any recognition that dispersants and inert fillers can be combined with a solid active ingredient to prepare a suspension having long term stability using a liquid medium in which the active ingredient has significant solubility.
A particularly attractive insecticide which displays these problems is Azinphos-methyl disclosed in U.S. Pat. No. 2,758,115 and more formally described as o,o-dimethyl s-[(4-oxo-1,2,3-benzotriazin-3(4H)-yl)methyl]phosphorodithioate: ##STR1## Formulations of this compound dissolved in aromatic petroleum fractions lose cold storage stability at temperatures of between 15.degree. C. and 0.degree. C. and tend to crystallize when combined with water at temperatures below about 15.degree. C. Furthermore, these formulations are incompatible when tank mixed with other pesticides registered with the EPA for the same crops such as BRAVO 6 fungicide marketed by Diamond Shamrock and BENLATE 50 fungicide marketed by DuPont, although this same compound when formulated as a wettable powder is compatible with these same pesticides. On the other hand, this wettable powder formulation is incompatible with such pesticides as BOLSTAR 6 insecticide marketed by Mobay Chemical Corporation and suitable for cotton as is Azinphos-methyl.